Network Working Group G. Clemm
Internet-Draft IBM
Updates: J. Crawford
draft-ietf-webdav-rfc2518bis IBM Research
(if approved) J. Reschke, Ed.
Intended status: Standards Track greenbytes
Expires: July 9, 2007 J. Whitehead
U.C. Santa Cruz
January 5, 2007
Binding Extensions to Web Distributed Authoring and Versioning (WebDAV)
draft-ietf-webdav-bind-16
Status of this Memo
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This specification defines bindings, and the BIND method for creating
multiple bindings to the same resource. Creating a new binding to a
resource causes at least one new URI to be mapped to that resource.
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Internet-Draft Binding Extensions to WebDAV January 20071. Introduction
This specification extends the WebDAV Distributed Authoring Protocol
([draft-ietf-webdav-rfc2518bis]) to enable clients to create new
access paths to existing resources. This capability is useful for
several reasons:
URIs of WebDAV-compliant resources are hierarchical and correspond to
a hierarchy of collections in resource space. The WebDAV Distributed
Authoring Protocol makes it possible to organize these resources into
hierarchies, placing them into groupings, known as collections, which
are more easily browsed and manipulated than a single flat
collection. However, hierarchies require categorization decisions
that locate resources at a single location in the hierarchy, a
drawback when a resource has multiple valid categories. For example,
in a hierarchy of vehicle descriptions containing collections for
cars and boats, a description of a combination car/boat vehicle could
belong in either collection. Ideally, the description should be
accessible from both. Allowing clients to create new URIs that
access the existing resource lets them put that resource into
multiple collections.
Hierarchies also make resource sharing more difficult, since
resources that have utility across many collections are still forced
into a single collection. For example, the mathematics department at
one university might create a collection of information on fractals
that contains bindings to some local resources, but also provides
access to some resources at other universities. For many reasons, it
may be undesirable to make physical copies of the shared resources on
the local server: to conserve disk space, to respect copyright
constraints, or to make any changes in the shared resources visible
automatically. Being able to create new access paths to existing
resources in other collections or even on other servers is useful for
this sort of case.
The BIND method defined here provides a mechanism for allowing
clients to create alternative access paths to existing WebDAV
resources. HTTP [RFC2616] and WebDAV [draft-ietf-webdav-rfc2518bis]
methods are able to work because there are mappings between URIs and
resources. A method is addressed to a URI, and the server follows
the mapping from that URI to a resource, applying the method to that
resource. Multiple URIs may be mapped to the same resource, but
until now there has been no way for clients to create additional URIs
mapped to existing resources.
BIND lets clients associate a new URI with an existing WebDAV
resource, and this URI can then be used to submit requests to the
resource. Since URIs of WebDAV resources are hierarchical, and
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correspond to a hierarchy of collections in resource space, the BIND
method also has the effect of adding the resource to a collection.
As new URIs are associated with the resource, it appears in
additional collections.
A BIND request does not create a new resource, but simply makes
available a new URI for submitting requests to an existing resource.
The new URI is indistinguishable from any other URI when submitting a
request to a resource. Only one round trip is needed to submit a
request to the intended target. Servers are required to enforce the
integrity of the relationships between the new URIs and the resources
associated with them. Consequently, it may be very costly for
servers to support BIND requests that cross server boundaries.
This specification is organized as follows. Section 1.1 defines
terminology used in the rest of the specification, while Section 2
overviews bindings. Section 3 defines the new properties needed to
support multiple bindings to the same resource. Section 4 specifies
the BIND method, used to create multiple bindings to the same
resource. Section 5 specifies the UNBIND method, used to remove a
binding to a resource. Section 6 specifies the REBIND method, used
to move a binding to another collection.
1.1. Terminology
The terminology used here follows and extends that in the WebDAV
Distributed Authoring Protocol specification
[draft-ietf-webdav-rfc2518bis].
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
This document uses XML DTD fragments ([XML]) as a notational
convention, using the rules defined in Section 17 of
[draft-ietf-webdav-rfc2518bis].
URI Mapping
A relation between an absolute URI and a resource. For an
absolute URI U and the resource it identifies R, the URI mapping
can be thought of as (U => R). Since a resource can represent
items that are not network retrievable, as well as those that are,
it is possible for a resource to have zero, one, or many URI
mappings. Mapping a resource to an "http" scheme URI makes it
possible to submit HTTP protocol requests to the resource using
the URI.
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Path Segment
Informally, the characters found between slashes ("/") in a URI.
Formally, as defined in Section 3.3 of [RFC3986].
Binding
A relation between a single path segment (in a collection) and a
resource. A binding is part of the state of a collection. If two
different collections contain a binding between the same path
segment and the same resource, these are two distinct bindings.
So for a collection C, a path segment S, and a resource R, the
binding can be thought of as C:(S -> R). Bindings create URI
mappings, and hence allow requests to be sent to a single resource
from multiple locations in a URI namespace. For example, given a
collection C (accessible through the URI
http://www.example.com/CollX), a path segment S (equal to
"foo.html"), and a resource R, then creating the binding C: (S ->
R) makes it possible to use the URI
http://www.example.com/CollX/foo.html to access R.
Collection
A resource that contains, as part of its state, a set of bindings
that identify internal member resources.
Internal Member URI
The URI that identifies an internal member of a collection, and
that consists of the URI for the collection, followed by a slash
character ('/'), followed by the path segment of the binding for
that internal member.
1.2. Method Preconditions and Postconditions
See Section 16 of [draft-ietf-webdav-rfc2518bis] for the definitions
of "precondition" and "postcondition".
2. Overview of Bindings
Bindings are part of the state of a collection. They define the
internal members of the collection, and the names of those internal
members.
Bindings are added and removed by a variety of existing HTTP methods.
A method that creates a new resource, such as PUT, COPY, and MKCOL,
adds a binding. A method that deletes a resource, such as DELETE,
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removes a binding. A method that moves a resource (e.g. MOVE) both
adds a binding (in the destination collection) and removes a binding
(in the source collection). The BIND method introduced here provides
a mechanism for adding a second binding to an existing resource.
There is no difference between an initial binding added by PUT, COPY,
or MKCOL, and additional bindings added with BIND.
It would be very undesirable if one binding could be destroyed as a
side effect of operating on the resource through a different binding.
In particular, the removal of one binding to a resource (e.g. with a
DELETE or a MOVE) MUST NOT disrupt another binding to that resource,
e.g. by turning that binding into a dangling path segment. The
server MUST NOT reclaim system resources after removing one binding,
while other bindings to the resource remain. In other words, the
server MUST maintain the integrity of a binding. It is permissible,
however, for future method definitions (e.g., a DESTROY method) to
have semantics that explicitly remove all bindings and/or immediately
reclaim system resources.
2.1. Bindings to Collections
Creating a new binding to a collection makes each resource associated
with a binding in that collection accessible via a new URI, and thus
creates new URI mappings to those resources but no new bindings.
For example, suppose a new binding CollY is created for collection C1
in the figure below. It immediately becomes possible to access
resource R1 using the URI /CollY/x.gif and to access resource R2
using the URI /CollY/y.jpg, but no new bindings for these child
resources were created. This is because bindings are part of the
state of a collection, and associate a URI that is relative to that
collection with its target resource. No change to the bindings in
Collection C1 is needed to make its children accessible using /CollY/
x.gif and /CollY/y.jpg.
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+-------------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+-------------------------+
| /
| /
| /
+------------------+
| Collection C1 |
| bindings: |
| x.gif y.jpg |
+------------------+
| \
| \
| \
+-------------+ +-------------+
| Resource R1 | | Resource R2 |
+-------------+ +-------------+
2.1.1. Bind loops
Bindings to collections can result in loops, which servers MUST
detect when processing "Depth: infinity" requests. It is sometimes
possible to complete an operation in spite of the presence of a loop.
For instance, a PROPFIND can still succeed if the server uses the new
status code 208 (Already Reported) defined in Section 7.1.
However, the 506 (Loop Detected) status code is defined in
Section 7.2 for use in contexts where an operation is terminated
because a loop was encountered.
2.2. URI Mappings Created by a new Binding
Suppose a binding from "Binding-Name" to resource R is to be added to
a collection, C. Then if C-MAP is the set of URIs that were mapped to
C before the BIND request, then for each URI "C-URI" in C-MAP, the
URI "C-URI/Binding-Name" is mapped to resource R following the BIND
request.
For example, if a binding from "foo.html" to R is added to a
collection C, and if the following URIs are mapped to C:
http://www.example.com/A/1/
http://example.com/A/one/
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then the following new mappings to R are introduced:
http://www.example.com/A/1/foo.html
http://example.com/A/one/foo.html
Note that if R is a collection, additional URI mappings are created
to the descendents of R. Also, note that if a binding is made in
collection C to C itself (or to a parent of C), an infinite number of
mappings are introduced.
For example, if a binding from "myself" to C is then added to C, the
following infinite number of additional mappings to C are introduced:
http://www.example.com/A/1/myself
http://www.example.com/A/1/myself/myself
...
and the following infinite number of additional mappings to R are
introduced:
http://www.example.com/A/1/myself/foo.html
http://www.example.com/A/1/myself/myself/foo.html
...
2.3. COPY and Bindings
As defined in Section 9.8 of [draft-ietf-webdav-rfc2518bis], COPY
causes the resource identified by the Request-URI to be duplicated,
and makes the new resource accessible using the URI specified in the
Destination header. Upon successful completion of a COPY, a new
binding is created between the last path segment of the Destination
header, and the destination resource. The new binding is added to
its parent collection, identified by the Destination header minus its
final segment.
The following figure shows an example: Suppose that a COPY is issued
to URI-3 for resource R (which is also mapped to URI-1 and URI-2),
with the Destination header set to URI-X. After successful
completion of the COPY operation, resource R is duplicated to create
resource R', and a new binding has been created which creates at
least the URI mapping between URI-X and the new resource (although
other URI mappings may also have been created).
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URI-1 URI-2 URI-3 URI-X
| | | |
| | | <---- URI Mappings ----> |
| | | |
+---------------------+ +------------------------+
| Resource R | | Resource R' |
+---------------------+ +------------------------+
It might be thought that a COPY request with "Depth: 0" on a
collection would duplicate its bindings, since bindings are part of
the collection's state. This is not the case, however. The
definition of Depth in [draft-ietf-webdav-rfc2518bis] makes it clear
that a "Depth: 0" request does not apply to a collection's members.
Consequently, a COPY with "Depth: 0" does not duplicate the bindings
contained by the collection.
If a COPY request causes an existing resource to be updated, the
bindings to that resource MUST be unaffected by the COPY request.
Using the preceding example, suppose that a COPY request is issued to
URI-X for resource R', with the Destination header set to URI-2. The
content and dead properties of resource R would be updated to be a
copy of those of resource R', but the mappings from URI-1, URI-2, and
URI-3 to resource R remain unaffected. If because of multiple
bindings to a resource, more than one source resource updates a
single destination resource, the order of the updates is server
defined.
If a COPY request would cause a new resource to be created as a copy
of an existing resource, and that COPY request has already created a
copy of that existing resource, the COPY request instead creates
another binding to the previous copy, instead of creating a new
resource.
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Internet-Draft Binding Extensions to WebDAV January 20072.4. DELETE and Bindings
When there are multiple bindings to a resource, a DELETE applied to
that resource MUST NOT remove any bindings to that resource other
than the one identified by the Request-URI. For example, suppose the
collection identified by the URI "/a" has a binding named "x" to a
resource R, and another collection identified by "/b" has a binding
named "y" to the same resource R. Then a DELETE applied to "/a/x"
removes the binding named "x" from "/a" but MUST NOT remove the
binding named "y" from "/b" (i.e. after the DELETE, "/y/b" continues
to identify the resource R).
When DELETE is applied to a collection, it MUST NOT modify the
membership of any other collection that is not itself a member of the
collection being deleted. For example, if both "/a/.../x" and
"/b/.../y" identify the same collection, C, then applying DELETE to
"/a" must not delete an internal member from C or from any other
collection that is a member of C, because that would modify the
membership of "/b".
If a collection supports the UNBIND method (see Section 5), a DELETE
of an internal member of a collection MAY be implemented as an UNBIND
request. In this case, applying DELETE to a Request-URI has the
effect of removing the binding identified by the final segment of the
Request-URI from the collection identified by the Request-URI minus
its final segment. Although [draft-ietf-webdav-rfc2518bis] allows a
DELETE to be a non-atomic operation, when the DELETE operation is
implemented as an UNBIND, the operation is atomic. In particular, a
DELETE on a hierarchy of resources is simply the removal of a binding
to the collection identified by the Request-URI.
2.5. MOVE and Bindings
When MOVE is applied to a resource, the other bindings to that
resource MUST be unaffected, and if the resource being moved is a
collection, the bindings to any members of that collection MUST be
unaffected. Also, if MOVE is used with Overwrite:T to delete an
existing resource, the constraints specified for DELETE apply.
If the destination collection of a MOVE request supports the REBIND
method (see Section 6), a MOVE of a resource into that collection MAY
be implemented as a REBIND request. Although
[draft-ietf-webdav-rfc2518bis] allows a MOVE to be a non-atomic
operation, when the MOVE operation is implemented as a REBIND, the
operation is atomic. In particular, applying a MOVE to a Request-URI
and a Destination URI has the effect of removing a binding to a
resource (at the Request-URI), and creating a new binding to that
resource (at the Destination URI). Even when the Request-URI
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identifies a collection, the MOVE operation involves only removing
one binding to that collection and adding another.
As an example, suppose that a MOVE is issued to URI-3 for resource R
below (which is also mapped to URI-1 and URI-2), with the Destination
header set to URI-X. After successful completion of the MOVE
operation, a new binding has been created which creates the URI
mapping between URI-X and resource R. The binding corresponding to
the final segment of URI-3 has been removed, which also causes the
URI mapping between URI-3 and R to be removed. If resource R were a
collection, old URI-3 based mappings to members of R would have been
removed, and new URI-X based mappings to members of R would have been
created.
>> Before Request:
URI-1 URI-2 URI-3
| | |
| | | <---- URI Mappings
| | |
+---------------------+
| Resource R |
+---------------------+
>> After Request:
URI-1 URI-2 URI-X
| | |
| | | <---- URI Mappings
| | |
+---------------------+
| Resource R |
+---------------------+
2.6. PROPFIND and Bindings
Consistent with [draft-ietf-webdav-rfc2518bis], the value of a dead
property MUST be independent of the number of bindings to its host
resource or of the path submitted to PROPFIND. On the other hand,
the behaviour for each live property depends on its individual
definition (for example, see [RFC3744], Section 5, paragraph 2).
2.7. Determining Whether Two Bindings Are to the Same Resource
It is useful to have some way of determining whether two bindings are
to the same resource. Two resources might have identical contents
and properties, but not be the same resource (e.g. an update to one
resource does not affect the other resource).
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The REQUIRED DAV:resource-id property defined in Section 3.1 is a
resource identifier, which MUST be unique across all resources for
all time. If the values of DAV:resource-id returned by PROPFIND
requests through two bindings are identical character by character,
the client can be assured that the two bindings are to the same
resource.
The DAV:resource-id property is created, and its value assigned, when
the resource is created. The value of DAV:resource-id MUST NOT be
changed. Even after the resource is no longer accessible through any
URI, that value MUST NOT be reassigned to another resource's DAV:
resource-id property.
Any method that creates a new resource MUST assign a new, unique
value to its DAV:resource-id property. For example, a PUT applied to
a null resource, COPY (when not overwriting an existing target) and
CHECKIN (see [RFC3253], Section 4.4) must assign a new, unique value
to the DAV:resource-id property of the new resource they create.
On the other hand, any method that affects an existing resource must
not change the value of its DAV:resource-id property. Specifically,
a PUT or a COPY that updates an existing resource must not change the
value of its DAV:resource-id property. A REBIND, since it does not
create a new resource, but only changes the location of an existing
resource, must not change the value of the DAV:resource-id property.
2.8. Discovering the Bindings to a Resource
An OPTIONAL DAV:parent-set property on a resource provides a list of
the bindings that associate a collection and a URI segment with that
resource. If the DAV:parent-set property exists on a given resource,
it MUST contain a complete list of all bindings to that resource that
the client is authorized to see. When deciding whether to support
the DAV:parent-set property, server implementers / administrators
should balance the benefits it provides against the cost of
maintaining the property and the security risks enumerated in
Sections 10.4 and 10.5.
3. Properties
The bind feature introduces the properties defined below.
A DAV:allprop PROPFIND request SHOULD NOT return any of the
properties defined by this document. This allows a binding server to
perform efficiently when a naive client, which does not understand
the cost of asking a server to compute all possible live properties,
issues a DAV:allprop PROPFIND request.
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Internet-Draft Binding Extensions to WebDAV January 20073.1. DAV:resource-id Property
The DAV:resource-id property is a REQUIRED property that enables
clients to determine whether two bindings are to the same resource.
The value of DAV:resource-id is a URI, and may use any registered URI
scheme that guarantees the uniqueness of the value across all
resources for all time (e.g. the urn:uuid: URN namespace defined in
[RFC4122] or the opaquelocktoken: URI scheme defined in
[draft-ietf-webdav-rfc2518bis]).
<!ELEMENT resource-id (href)>
3.2. DAV:parent-set Property
The DAV:parent-set property is an OPTIONAL property that enables
clients to discover what collections contain a binding to this
resource (i.e. what collections have that resource as an internal
member). It contains an of href/segment pair for each collection
that has a binding to the resource. The href identifies the
collection, and the segment identifies the binding name of that
resource in that collection.
A given collection MUST appear only once in the DAV:parent-set for
any given binding, even if there are multiple URI mappings to that
collection.
<!ELEMENT parent-set (parent)*>
<!ELEMENT parent (href, segment)>
<!ELEMENT segment (#PCDATA)>
<!-- PCDATA value: segment, as defined in Section 3.3
[RFC3986] -->
3.2.1. Example for DAV:parent-set property
For example, if collection C1 is mapped to both /CollX and /CollY,
and C1 contains a binding named "x.gif" to a resource R1, then either
[/CollX, x.gif] or [/CollY, x.gif] can appear in the DAV:parent-set
of R1, but not both. But if C1 also had a binding named "y.gif" to
R1, then there would be two entries for C1 in the DAV:binding-set of
R1 (i.e. both [/CollX, x.gif] and [/CollX, y.gif] or, alternatively,
both [/CollY, x.gif] and [/CollY, y.gif]).
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+-------------------------+
| Root Collection |
| bindings: |
| CollX CollY |
+-------------------------+
| /
| /
| /
+-----------------+
| Collection C1 |
| bindings: |
| x.gif y.gif |
+-----------------+
| |
| |
| |
+--------------+
| Resource R1 |
+--------------+
In this case, one possible value for DAV:parent-set property on
"/CollX/x.gif" would be:
<parent-set xmlns="DAV:">
<parent>
<href>/CollX</href>
<segment>x.gif</segment>
</parent>
<parent>
<href>/CollX</href>
<segment>y.gif</segment>
</parent>
</parent-set>
4. BIND Method
The BIND method modifies the collection identified by the Request-
URI, by adding a new binding from the segment specified in the BIND
body to the resource identified in the BIND body.
If a server cannot guarantee the integrity of the binding, the BIND
request MUST fail. Note that it is especially difficult to maintain
the integrity of cross-server bindings. Unless the server where the
resource resides knows about all bindings on all servers to that
resource, it may unwittingly destroy the resource or make it
inaccessible without notifying another server that manages a binding
to the resource. For example, if server A permits creation of a
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binding to a resource on server B, server A must notify server B
about its binding and must have an agreement with B that B will not
destroy the resource while A's binding exists. Otherwise server B
may receive a DELETE request that it thinks removes the last binding
to the resource and destroy the resource while A's binding still
exists. The precondition DAV:cross-server-binding is defined below
for cases where servers fail cross-server BIND requests because they
cannot guarantee the integrity of cross-server bindings.
By default, if there already is a binding for the specified segment
in the collection, the new binding replaces the existing binding.
This default binding replacement behavior can be overridden using the
Overwrite header defined in Section 10.6 of
[draft-ietf-webdav-rfc2518bis].
If a BIND request fails, the server state preceding the request MUST
be restored. This method is unsafe and idempotent (see [RFC2616],
Section 9.1).
Marshalling:
The request MAY include an Overwrite header.
The request body MUST be a DAV:bind XML element.
<!ELEMENT bind (segment, href)>
If the request succeeds, the server MUST return 201 (Created) when
a new binding was created and 200 (OK) when an existing binding
was replaced.
If a response body for a successful request is included, it MUST
be a DAV:bind-response XML element. Note that this document does
not define any elements for the BIND response body, but the DAV:
bind-response element is defined to ensure interoperability
between future extensions that do define elements for the BIND
response body.
<!ELEMENT bind-response ANY>
Preconditions:
(DAV:bind-into-collection): The Request-URI MUST identify a
collection.
(DAV:bind-source-exists): The DAV:href element MUST identify a
resource.
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(DAV:binding-allowed): The resource identified by the DAV:href
supports multiple bindings to it.
(DAV:cross-server-binding): If the resource identified by the DAV:
href element in the request body is on another server from the
collection identified by the Request-URI, the server MUST support
cross-server bindings.
(DAV:name-allowed): The name specified by the DAV:segment is
available for use as a new binding name.
(DAV:can-overwrite): If the collection already contains a binding
with the specified path segment, and if an Overwrite header is
included, the value of the Overwrite header MUST be "T".
(DAV:cycle-allowed): If the DAV:href element identifies a
collection, and if the Request-URI identifies a collection that is
a member of that collection, the server MUST support cycles in the
URI namespace.
(DAV:locked-update-allowed): If the collection identified by the
Request-URI is write-locked, then the appropriate token MUST be
specified in an If request header.
(DAV:locked-overwrite-allowed): If the collection already contains
a binding with the specified path segment, and if that binding is
protected by a write-lock, then the appropriate token MUST be
specified in an If request header.
Postconditions:
(DAV:new-binding): The collection MUST have a binding that maps
the segment specified in the DAV:segment element in the request
body, to the resource identified by the DAV:href element in the
request body.
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>> Request:
BIND /CollY HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:bind xmlns:D="DAV:">
<D:segment>bar.html</D:segment>
<D:href>http://www.example.com/CollX/foo.html</D:href>
</D:bind>
>> Response:
HTTP/1.1 201 Created
The server added a new binding to the collection,
"http://www.example.com/CollY", associating "bar.html" with the
resource identified by the URI
"http://www.example.com/CollX/foo.html". Clients can now use the URI
"http://www.example.com/CollY/bar.html" to submit requests to that
resource.
5. UNBIND Method
The UNBIND method modifies the collection identified by the Request-
URI, by removing the binding identified by the segment specified in
the UNBIND body.
Once a resource is unreachable by any URI mapping, the server MAY
reclaim system resources associated with that resource. If UNBIND
removes a binding to a resource, but there remain URI mappings to
that resource, the server MUST NOT reclaim system resources
associated with the resource.
If an UNBIND request fails, the server state preceding the request
MUST be restored. This method is unsafe and idempotent (see
[RFC2616], Section 9.1).
Marshalling:
The request body MUST be a DAV:unbind XML element.
<!ELEMENT unbind (segment)>
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If the request succeeds, the server MUST return 200 (OK) when the
binding was successfully deleted.
If a response body for a successful request is included, it MUST
be a DAV:unbind-response XML element. Note that this document
does not define any elements for the UNBIND response body, but the
DAV:unbind-response element is defined to ensure interoperability
between future extensions that do define elements for the UNBIND
response body.
<!ELEMENT unbind-response ANY>
Preconditions:
(DAV:unbind-from-collection): The Request-URI MUST identify a
collection.
(DAV:unbind-source-exists): The DAV:segment element MUST identify
a binding in the collection identified by the Request-URI.
(DAV:locked-update-allowed): If the collection identified by the
Request-URI is write-locked, then the appropriate token MUST be
specified in the request.
(DAV:protected-url-deletion-allowed): If the binding identified by
the segment is protected by a write-lock, then the appropriate
token MUST be specified in the request.
Postconditions:
(DAV:binding-deleted): The collection MUST NOT have a binding for
the segment specified in the DAV:segment element in the request
body.
(DAV:lock-deleted): If the internal member URI of the binding
specified by the Request-URI and the DAV:segment element in the
request body was protected by a write-lock at the time of the
request, that write-lock must have been deleted by the request.
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Internet-Draft Binding Extensions to WebDAV January 20075.1. Example: UNBIND
>> Request:
UNBIND /CollX HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:unbind xmlns:D="DAV:">
<D:segment>foo.html</D:segment>
</D:unbind>
>> Response:
HTTP/1.1 200 OK
The server removed the binding named "foo.html" from the collection,
"http://www.example.com/CollX". A request to the resource named
"http://www.example.com/CollX/foo.html" will return a 404 (Not Found)
response.
6. REBIND Method
The REBIND method removes a binding to a resource from a collection,
and adds a binding to that resource into the collection identified by
the Request-URI. The request body specifies the binding to be added
(segment) and the old binding to be removed (href). It is
effectively an atomic form of a MOVE request, and MUST be treated the
same way as MOVE for the purpose of determining access permissions.
If a REBIND request fails, the server state preceding the request
MUST be restored. This method is unsafe and idempotent (see
[RFC2616], Section 9.1).
Marshalling:
The request MAY include an Overwrite header.
The request body MUST be a DAV:rebind XML element.
<!ELEMENT rebind (segment, href)>
If the request succeeds, the server MUST return 201 (Created) when
a new binding was created and 200 (OK) when an existing binding
was replaced.
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If a response body for a successful request is included, it MUST
be a DAV:rebind-response XML element. Note that this document
does not define any elements for the REBIND response body, but the
DAV:rebind-response element is defined to ensure interoperability
between future extensions that do define elements for the REBIND
response body.
<!ELEMENT rebind-response ANY>
Preconditions:
(DAV:rebind-into-collection): The Request-URI MUST identify a
collection.
(DAV:rebind-source-exists): The DAV:href element MUST identify a
resource.
(DAV:cross-server-binding): If the resource identified by the DAV:
href element in the request body is on another server from the
collection identified by the Request-URI, the server MUST support
cross-server bindings.
(DAV:name-allowed): The name specified by the DAV:segment is
available for use as a new binding name.
(DAV:can-overwrite): If the collection already contains a binding
with the specified path segment, and if an Overwrite header is
included, the value of the Overwrite header MUST be "T".
(DAV:cycle-allowed): If the DAV:href element identifies a
collection, and if the Request-URI identifies a collection that is
a member of that collection, the server MUST support cycles in the
URI namespace.
(DAV:locked-update-allowed): If the collection identified by the
Request-URI is write-locked, then the appropriate token MUST be
specified in the request.
(DAV:protected-url-modification-allowed): If the collection
identified by the Request-URI already contains a binding with the
specified path segment, and if that binding is protected by a
write-lock, then the appropriate token MUST be specified in the
request.
(DAV:locked-source-collection-update-allowed): If the collection
identified by the parent collection prefix of the DAV:href URI is
write-locked, then the appropriate token MUST be specified in the
request.
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(DAV:protected-source-url-deletion-allowed): If the DAV:href URI
is protected by a write lock, then the appropriate token MUST be
specified in the request.
Postconditions:
(DAV:new-binding): The collection MUST have a binding that maps
the segment specified in the DAV:segment element in the request
body, to the resource that was identified by the DAV:href element
in the request body.
(DAV:binding-deleted): The URL specified in the DAV:href element
in the request body MUST NOT be mapped to a resource.
(DAV:lock-deleted): If the URL specified in the DAV:href element
in the request body was protected by a write-lock at the time of
the request, that write-lock must have been deleted by the
request.
6.1. Example: REBIND
>> Request:
REBIND /CollX HTTP/1.1
Host: www.example.com
Content-Type: application/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:rebind xmlns:D="DAV:">
<D:segment>foo.html</D:segment>
<D:href>http://www.example.com/CollY/bar.html</D:href>
</D:rebind>
>> Response:
HTTP/1.1 200 OK
The server added a new binding to the collection,
"http://www.example.com/CollX", associating "foo.html" with the
resource identified by the URI
"http://www.example.com/CollY/bar.html", and removes the binding
named "bar.html" from the collection identified by the URI
"http://www.example.com/CollY". Clients can now use the URI
"http://www.example.com/CollX/foo.html" to submit requests to that
resource, and requests on the URI
"http://www.example.com/CollY/bar.html" will fail with a 404 (Not
Found) response.
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Note that the 208 status will only occur for "Depth: infinity"
requests, and that it is of particular importance when the multiple
collection bindings cause a bind loop as discussed in Section 2.2.
A client can request the DAV:resource-id property in a PROPFIND
request to guarantee that they can accurately reconstruct the binding
structure of a collection with multiple bindings to a single
resource.
For backward compatibility with clients not aware of the 208 status
code appearing in multistatus response bodies, it SHOULD NOT be used
unless the client has signalled support for this specification using
the "DAV" request header (see Section 8.2). Instead, a 506 status
should be returned when a binding loop is discovered. This allows
the server to return the 506 as the top level return status, if it
discovers it before it started the response, or in the middle of a
multistatus, if it discovers it in the middle of streaming out a
multistatus response.
7.1.1. Example: PROPFIND by bind-aware client
For example, consider a PROPFIND request on /Coll (bound to
collection C), where the members of /Coll are /Coll/Foo (bound to
resource R) and /Coll/Bar (bound to collection C).
>> Request:
PROPFIND /Coll/ HTTP/1.1
Host: www.example.com
Depth: infinity
DAV: bind
Content-Type: application/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:prop>
<D:displayname/>
<D:resource-id/>
</D:prop>
</D:propfind>
>> Response:
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Internet-Draft Binding Extensions to WebDAV January 20077.1.2. Example: PROPFIND by non-bind-aware client
In this example, the client isn't aware of the 208 status code
introduced by this specification. As the "Depth: infinity" PROPFIND
request would cause a loop condition, the whole request is rejected
with a 506 status.
>> Request:
PROPFIND /Coll/ HTTP/1.1
Host: www.example.com
Depth: infinity
Content-Type: application/xml; charset="utf-8"
Content-Length: xxx
<?xml version="1.0" encoding="utf-8" ?>
<D:propfind xmlns:D="DAV:">
<D:prop> <D:displayname/> </D:prop>
</D:propfind>
>> Response:
HTTP/1.1 506 Loop Detected
7.2. 506 Loop Detected
The 506 (Loop Detected) status code indicates that the server
terminated an operation because it encountered an infinite loop while
processing a request with "Depth: infinity". This status indicates
that the entire operation failed.
8. Capability discovery8.1. OPTIONS method
If the server supports bindings, it MUST return the compliance class
name "bind" as a field in the "DAV" response header (see
[draft-ietf-webdav-rfc2518bis], Section 10.1) from an OPTIONS request
on any resource implemented by that server. A value of "bind" in the
"DAV" header MUST indicate that the server supports all MUST level
requirements and REQUIRED features specified in this document.
8.2. 'DAV' request header
Clients SHOULD signal support for all MUST level requirements and
REQUIRED features by submitting a "DAV" request header containing the
compliance class name "bind". In particular, the client MUST
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understand the 208 status code defined in Section 7.1.
9. Relationship to WebDAV Access Control Protocol
BIND and REBIND behave the same as MOVE with respect to the DAV:acl
property (see [RFC3744], Section 7.3).
10. Security Considerations
This section is provided to make WebDAV implementors aware of the
security implications of this protocol.
All of the security considerations of HTTP/1.1 and the WebDAV
Distributed Authoring Protocol specification also apply to this
protocol specification. In addition, bindings introduce several new
security concerns and increase the risk of some existing threats.
These issues are detailed below.
10.1. Privacy Concerns
In a context where cross-server bindings are supported, creating
bindings on a trusted server may make it possible for a hostile agent
to induce users to send private information to a target on a
different server.
10.2. Bind Loops
Although bind loops were already possible in HTTP 1.1, the
introduction of the BIND method creates a new avenue for clients to
create loops accidentally or maliciously. If the binding and its
target are on the same server, the server may be able to detect BIND
requests that would create loops. Servers are required to detect
loops that are caused by bindings to collections during the
processing of any requests with "Depth: infinity".
10.3. Bindings, and Denial of Service
Denial of service attacks were already possible by posting URIs that
were intended for limited use at heavily used Web sites. The
introduction of BIND creates a new avenue for similar denial of
service attacks. If cross-server bindings are supported, clients can
now create bindings at heavily used sites to target locations that
were not designed for heavy usage.
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Internet-Draft Binding Extensions to WebDAV January 200710.4. Private Locations May Be Revealed
If the DAV:parent-set property is maintained on a resource, the
owners of the bindings risk revealing private locations. The
directory structures where bindings are located are available to
anyone who has access to the DAV:parent-set property on the resource.
Moving a binding may reveal its new location to anyone with access to
DAV:parent-set on its resource.
10.5. DAV:parent-set and Denial of Service
If the server maintains the DAV:parent-set property in response to
bindings created in other administrative domains, it is exposed to
hostile attempts to make it devote resources to adding bindings to
the list.
11. Internationalization Considerations
All internationalization considerations mentioned in
[draft-ietf-webdav-rfc2518bis] also apply to this document.
12. IANA Considerations
There are no IANA considerations related to this specification.
13. Acknowledgements
This document is the collaborative product of the authors and Tyson
Chihaya, Jim Davis, Chuck Fay and Judith Slein. This draft has
benefited from thoughtful discussion by Jim Amsden, Peter Carlson,
Steve Carter, Ken Coar, Ellis Cohen, Dan Connolly, Bruce Cragun,
Spencer Dawkins, Mark Day, Rajiv Dulepet, David Durand, Lisa
Dusseault, Stefan Eissing, Roy Fielding, Yaron Goland, Joe
Hildebrand, Fred Hitt, Alex Hopmann, James Hunt, Marcus Jager, Chris
Kaler, Manoj Kasichainula, Rohit Khare, Brian Korver, Daniel
LaLiberte, Steve Martin, Larry Masinter, Jeff McAffer, Surendra
Koduru Reddy, Max Rible, Sam Ruby, Bradley Sergeant, Nick Shelness,
John Stracke, John Tigue, John Turner, Kevin Wiggen, and other
members of the WebDAV working group.
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This is incorrect in that it implies that the "lock root" is a
resource, not a URL
(<http://ietf.osafoundation.org:8080/bugzilla/show_bug.cgi?id=251>).
However, should a directly locked resource have multiple bindings,
only the one used in the Request-URI of the LOCK request will be the
protected from changes of clients not supplying the lock token.
A correct description would be:
A LOCK request to an existing resource will create a lock on the
resource identified by the Request-URI, provided the resource is
not already locked with a conflicting lock. The Request-URI
becomes the root of the lock.
Note that this change makes the description consistent with the
definition of the DAV:lockroot XML element in Section 14.12 of
[draft-ietf-webdav-rfc2518bis].
The authors of this specification recommend that future revisions of
[draft-ietf-webdav-rfc2518bis] will update the description as
suggested above.
Appendix B. Change Log (to be removed by RFC Editor before publication)B.1. Since draft-ietf-webdav-bind-02
Add and resolve issues "2.3_COPY_SHARED_BINDINGS" and
"2.3_MULTIPLE_COPY". Add issue "5.1_LOOP_STATUS" and proposed
resolution, but keep it open. Add issues "ED_references" and
"4_507_status". Started work on index. Rename document to "Binding
Extensions to Web Distributed Authoring and Versioning (WebDAV)".
Rename "References" to "Normative References". Close issue
"ED_references". Close issue "4_507_status".
B.2. Since draft-ietf-webdav-bind-03
Add and close issues "9.2_redirect_loops", "ED_authors" and
"ED_updates". Add section about capability discovery (DAV header).
Close issues "5.1_LOOP_STATUS". Add and resolve new issue
"5.1_506_STATUS_STREAMING". Update XML spec reference. Add issue
"locking" and resolve as invalid.
B.3. Since draft-ietf-webdav-bind-04
Add and close issues "6_precondition_binding_allowed" and
"6_lock_behaviour". Add mailing list and issues list pointers to
front.
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